1. Field of the Invention
This invention relates to electrolytic plating and more particularly, to control of the additive content in an electroplating solution.
2. Description of the Prior Art
Electroplating is a complex process involving multiple ingredients in a plating bath. It is important that the concentration of several of the ingredients be kept within close tolerances in order to obtain a high quality deposit. In some cases, chemical analysis of individual solution constituents can be made regularly (such as pH measurement for acid content), and additions made as required. However, other addition agents such as brighteners, leveling agents, suppressants, etc. together with impurities cannot be individually analyzed on an economical or timely basis by a commercial plating shop. Their initial concentration is low and their quantitative analysis is complicated and subject to error.
A prior art method for controlling such ingredients in an electroplating bath is to make regular additions of particular ingredients based upon empirical rules established by experience. However, depletion of particular ingredients is not always constant with time or with bath used. Consequently, the concentration of the ingredient in the bath eventually diminishes or increases to a level where it is out of the range of acceptable tolerance. If the additive content does go out of range, the quality of the metal deposit suffers and the deposit may be dull in appearance and/or brittle or powdery in structure.
Another prior art method for plating bath control is to plate articles or samples and visually evaluate the plating quality to determine if the bath is performing satisfactorily. In standard Hull Cell and "Bone Pattern" tests, a specially shaped test specimen is plated and then evaluated to determine the quality of the deposit along the shape. This is a time consuming test which gives at best a rough approximation of the concentration of the constituents of the bath.
The electroplating of through-hole interconnections in the manufacture of multilayer printed circuit boards is an example of a use of an electroplated metal where high quality plating is required. It is known that the concentration of the addition agent within the plating solution must be maintained in low concentration (typically less than 100 parts per million parts of solution - ppm) in order to obtain acceptable deposits on printed circuit boards. This must be done to maintain proper mechanical properties for resistance to thermal stresses encountered during manufacture and use and to assure the proper thickness of the deposit in the through-holes. The concentration of the addition agent fluctuates because of oxidation at the anode, reduction and hydrogen retention at the cathode, and chemical degradation. When the additive level is insufficient, deposits are burnt and powdery in appearance whereas excessive addition agent induces brittleness and non-uniform deposition. Hull Cell tests, Bone Pattern Tests, and Pencil Tests, combined with periodic additions of fresh additive, were the methods used to maintain a controlled concentration of the additive until recently. These methods were unreliable and circuit board quality suffered as a consequence of these unreliable methods.
A more recent method for evaluating the quality of an electroplating bath was disclosed in U.S. Pat. No. 4,132,605 to Tench (hereafter the Tench patent) incorporated herein by reference. In accordance with the procedures of the Tench patent, the potential of a working electrode is swept through a voltammetric cycle, including a metal plating range and a metal stripping range, for at least two baths of known plating quality and an additional bath whose quality or concentration of additives is to be evaluated. The integrated or peak current utilized during the metal stripping range is correlated with the quality of the bath of known quality. The integrated or peak current utilized to strip the metal in the bath of unknown quality is compared to the correlation and its quality evaluated. In a preferred embodiment of said patent, the potential of an inert working electrode is swept by a function generator through the voltammetric cycle. A counter electrode immersed in the plating bath is coupled in series with a function generator and a coulometer to measure the charge from the working electrode during the stripping portion of the cycle.
An improvement to the method disclosed in said U.S. Pat. No. 4,132,605 is described by Tench and White, in the J. Electrochem. Soc., Electrochemical Science and Technology, April, 1985, pp. 831-834, (hereafter the Tench publication) incorporated herein by reference. In accordance with the Tench publication, contaminant build up in the copper plating bath affects the copper deposition rate and thus interferes with additive analysis. The Tench publication teaches that rather than the continuous sweep cycle utilized in the above reference patent, a method be used involving sequentially pulsing the electrode between appropriate plating, stripping, cleaning, and equilibrium potentials whereby the electrode surface is maintained in a clean and reproducible state. Stated otherwise, where the process of the Tench patent involves a continuous voltammetric sweep between about -600 millivolts and +1,000 millivolts versus a working electrode and back over a period of about 1 minute, the Tench publication pulses the potential, for example at -250 mV for 2 seconds to plate, +200 mV for a time sufficient to strip, +1,600 mV to clean for seconds, +425 mV for 5 seconds to equilibrate, all potentials referenced to a saturated Calomel electrode, after which the cycle is repeated until the difference between successive results are within a predetermined value, for example, within 2% of one another.
The procedure of the Tench publication provides some improvement over the procedure of the Tench patent, but during continuous use of an electroplating bath and following successive analyses, contaminants build up on the electrodes and analysis sensitivity is lost.